And now for a truly weird creature - but one which is very much a real animal.  The horseshoe crab is about as alien as an animal can get.  The family Limulidae consists of four living varieties of horseshoe crab: one native to Japan, two native to India, and one native to the eastern United States.  

I have an early childhood memory of being startled and terrified by the sight of a flipped-over horseshoe crab.  I can't remember who flipped it over or why, but I do remember that I had thought it was an inanimate object like a big seashell.  An understandable mistake, particularly for a small child!  The sight of all those legs flibberty gibbeting around - that sight has really stuck with me.

"All those legs" in this case is ten, plus a pair of pincers.  Despite the fact that crabs also have ten legs plus a pair of pincers, and despite the fact that it's called a "horseshoe crab," the horseshoe crab is more closely related to spiders than to crabs.  They are the closest living relatives of trilobites, those wee little beasties you may recognize from their fossils, which are fairly common.

Horseshoe crabs have remained almost unchanged since their first appearance in the fossil record, 400 million years ago in the Devonian.  In other words, horseshoe crabs were just settling down and getting comfortable when fish first evolved legs and began to crawl out onto dry land.  They predate such recent novelties as flowering plants and winged insects.

Horseshoe crabs scuttle along the continental shelf, eating mollusks and worms.  They emerge every spring to clamber up onto beaches for mating, where they can breathe quite well as long as their gills are kept damp.  After laying their eggs in the sand, the horseshoe crabs return to the depths, where they can live for up to 20 years.

The blood of the horseshoe crab is blue, because it is based on copper to transport oxygen.  As opposed to the blood of most other animals, which is red because it's based on iron. (I find it funny to think that this makes their blood blue, since the copper-based blood of Vulcans was always said to be green, and guess who didn't do their science homework, way back when?)  

This blood has an interesting property, which is that it coagulates in response to bacteria.  Horseshoe crabs have only one type of blood cell, which is responsible for both carrying oxygen and defending the crab against illness.  Its response to encountering a pathogen is to clot up.

This lets humans use the blood of horseshoe crabs to test for pathogens ourselves. The blood can be safely withdrawn from horseshoe crab "blood donors," just as it is drawn from humans.  It can then be used in a variety of medical applications, including the International Space Station, to test for the presence of bacterial contaminants.  It is also used to test for biological contaminants in vaccines and medicine.

The horseshoe crab: Ugly and more than a little creepy?  Yes.  Unique?  Definitely!